Effect of Axial Velocity and Initial Flux on Flux Decline of Cellulose Acetate Membranes in Hyperfdtration of Primary Sewage Effluents David G. Thomas1 and William R. Mixon Oak Ridge National Laboratory, Oak Ridge, T N 37830
Axial velocities greater than 10-1 5 ft/sec were shown to ameliorate flux decline when using hyperfiltration to treat primary sewage plant effluent with cellulose acetate membranes having initial ( l / 2 hr after startup) fluxes of 40-150 gal/ft2.day. For axial velocities from 4 to 30 ft/sec, the flux decline parameter b[b = - (A log flux/A log time)] was proportional to the square root of the initial flux. When the axial velocity was greater than the critical value of 10-1 5 ft/sec the value of b was 99% rejection of bacteria, a reduction in biological oxygen demand (BOD) from a range of 8-26 ppm to 1 ppm or less, and complete removal of odor and turbidity (Bregman, 1970). Aside from the chemical problems of tailoring membranes to improve their rejection capability for such materials as ammonia, phenols, and detergents, one of the principal problems in sewage treatment applications has been the marked flus declines often observed with cellulose acet'ate membranes (Bregman, 1970). For instance, in oiie study (Merten and Bray, 1966)) only modest flux declines (-2070 in 1600 hr) occurred when the initial flus was -2 gal/ft2.day; subst,antial flux declines occurred (with a different membrane) when the initial flux was -10 gal/'ft2.day. X surprising result of hIert,en and Bray's (1966) tests n'ns that flux from a membrane wit'h an initially high flus eventually declined to a lower value than the flux from a membrane with an initially low flux operated for the same length of time (-600 hr). .\melioration of the flux decline problem has been approached in a variety of ways: aeration of primary effluent, carbon pretreatment of feed, and chemical cleaning with such agents as sodium hydrosulfite, acet'ic acid, and enzymes (Ihegman, 1970). I n studies with untreated lake water as feed, Sheppard and Thomas (19iOa) have shown that proper hydrodynamic conditions could minimize flux decline wit'h high-fouling potential feeds. The object of the present study was to esplore the effect of axial velocity 011 flus decline in hj-perfiltration of primary sewage effluent using cellulose acetate membranes. Primary effluents were used since in earlier studies with dynamic niembraiies (Kraus, 1970) there seemed to be little additional difficulties oyer those experienced with secondary effluents, I
_
Tu whom correapondence bhould be addreased.
and it seemed desirable to determine if similar advantages could be realized with cellulose acetate membranes. Eauinment and Procedure
Feed for these tests was effluent from t'he primary stage of the Oak Ridge East Treatment Plant. Although composition of the effluents varied considerably, typical values were: 50-100 ppm for organic carbon, 35-60 ppm for inorganic carbon, 30-60 ppm for CaZ+ Mgz+, and 20-50 ppm for total phosphate. T o facilitate analysis, sufficient, salt was added to make the feed -0.01J1 in NaCI. The loop used in the tests was identical with that described earlier (Kraus, 1970) and was located a t the sewage plant where a booster pump supplied feed from downstream of t'he primary settling tanks. Contents of the loop were circulated a t high axial velocity past the cellulose acetate membranes with a canned rotor pump rated for 120 gpm a t 294-ft head. The pump discharged through a heat exchanger to a manifold connected to t'hree test sections; the flow rate through each test section was monitored by a Venturi. Each test section consisted of two 6-in.-long, 5/s-i11.-o.d. porous stainless steel tubes housed in a transparent pressure jacket 18 in. long and 7 j in. ~ i.d. Xembranes were sealed on the outside of the porous stainless steel t'ubes with pressuresensitive t'ape. Since each 6-in.-long section of porous tubing could be sampled separat'ely, i t \vas possible to compare the performance of two different membranes under substantially the same flow conditions. Two different membranes were used in these tests: XerojetGeneral 6% permeation sheet membrane with an initial flux of -40 gal/ft*.day and 94% rejection a t 800-psig applied pressure and Eastman HT-00 cellulose acetate with a n initial flux of 150-200 galjft2.day a t 800-psig applied pressure and substantially zero rejections when the feed is XaC1 dissolved in demineralized water. -111 test.s wit,h these niembranes were carried out a t a system pressure of 1000 psig. Chloride analysis was performed amperometrically with a Buchler-Cotlove chloridometer; Ca2+ N g 2 + were measured together by EDTA\titration (Eriochrome Black T indicator) ; phosphate was determined by a colorimetric method using aminonaphthosulfonic acid; turbidity was determined with a Hach
+
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Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , NO. 3, 1972
339
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102
2
5
103
OPERATING TIME (hrl
Figure 1 . Effect of time and axial velocity on flux and rejection of cellulose acetate membranes with primary sewage plant effluent as feed
10
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a
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L
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Figure 2. Effect of axial velocity on flux and rejection of aerojet 6% permeation and Eastman HT-00 cellulose acetate membranes after 100 hr exposure to effluent from primary sewage plant Pressure = 1000 psig
2100 turbidimeter; and organic carbon was obtained from the difference between total and inorganic carbon determined with a Beckman 915 analyzer. Results
Three membrane housings mere used in most tests, each exposing two different membranes to a different velocity in the range 4-30 ft/sec. Duration of the tests n a s usually 150-200 hr. Results from a typical test are shown in Figure 1 as observed chloride rejection and flux as a function of time. Separate plots are shon n for the Aerojet and Eastman 340 Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
membranes, which in this instance were exposed to axial velocities of 4.0, 7.6, and 15.0 ft/sec. I n these tests, membranes were exposed initially to city water feed (spiked to 0.011\1 XaCl) to establish a base line for comparison with performance on exposure to primary effluent. During this initial period the rejection of the Aerojet membrane for C1- was 85-90%, and the flux was 40-55 gal/ft2.day, essentially independent of axial velocity. Rejection of the Eastman membrane was less than 10% (as expected), and the flux was 180-250 gal/ft*.day, also essentially independent of velocity. K h e n the feed was switched from city water to primary sewage effluent, rejection of both membranes increased; the -\erojet membrane rejection went from 8 5 9 0 % to 95%, the Eastman membrane rejection went from less than 10% to 16-18% nithin the first half hour of exposure to primary effluent feed and then to -407, in the next several hours. I n the initial half hour, there was a marked decrease in flux with the high-flux membrane but little change in the flux with the lon er-flux membrane. During the next 100 hr of this run, there was only a modest flux decline with both membranes operating a t the highest velocity used in this test (15 ft/sec), from 40 to 32 and from 105 to 90 gal/ft2.day for the Xerojet and Eastman membranes, respectively. Hovr-ever, there was marked flux decline for the membranes operating a t 7.6 and 4.0 ft/sec, from -40 to -4 gal/ft2'day. The effect of axial velocity on flux and rejection after 100 hr exposure to effluent from primary sewage treatment is shown for all runs in Figure 2. (Points with the same symbol lvere obtained simultaneously during the same run, and the runs were made during the period February-July 1970.) A striking feature of these results is the strong dependence of flux on velocity a t low velocities and the small effect of velocity on flux a t high velocities. With the Eastman membrane (with an average flux hr after startup with city water of -150 gal/ft2. day) the critical velocity for flux independent of velocity n a s -14 ft/sec while with the herojet membrane (with an average flux '/z hr after startup with city water of -50 gal/ftZ,day) the critical velocity was -10 ft/sec. Although there was considerable scatter in observed rejec tions, it appears that velocity had a strong effect on the rejec-
I02
Table I. Typical Rejections Observed with Cellulose Acetate Membranes and Primary Sewage Plant Effluent Feeds
5
Obad reiection
Membrane
Eastman HT-00
Axial velocity, ft/sec
30 15.7 7.6 4.0 herojet 6% 30 permeation 15.7 7.6 4.0
Ca2f CI-
46 42 26 40 99.5 99.5 87.9 79.7
Organic Turbidcorbon ity
83 74 81 74 93 91 86 86
98 91 98 92 99.3 98.2 99.3 94.8
+
Mg2+
~ 0 ~ 3 -
61 29 44 54 96 99.7 99.7 99.6
91 69 72 78 99.7 97.8 98.5 97.8
tion of the Aerojet 6% permeation membrane. The nominal value of 94y0 was observed a t axial velocities of 10-15 ft/sec where the flus mas -25 gal/ft2.day. At higher velocities, the observed rejection was greater than 99% (flus -25 gal/ft2. day), while a t lower velocities the observed rejection was as low as 80% flu^ -4 gal/ft?.day). The increase in rejection (from nominal 94 to >99%) may have been due to formation of a thin dynamic membrane from constituents in the sewage feed (as has been described b y Kraus, 1970) or to plugging of defects in the membrane. The decrease in rejection may have been due to the deposition of a sufficiently thick layer on the membrane that concentration polarization was markedly increased (Sheppard and Thomas, 1971) as well as the decrease in flus (Sheppard and Thomas, 197Oa). Apparently sewage constituents also formed a dynamic membrane on the Eastman cellulose acetate (which is nominally zero-rejecting), resulting in a n average rejection of -40Yc independent of velocity. I n prior studies (Kraus, 1970) primary sewage effluent was also observed to form a salt-rejecting membrane on a n Acropor film with a n observed rejection of 40-45% and a flux of 25 gal/ft2.day after 100 hr when the axial velocity was 30 ft/sec and the applied pressure was 1000 psig. Although membrane rejection was followed routinely by chloride analysis, occasional samples were analyzed for r e jectioii of organic carbon, turbidity, e a 2 + Mg2+,and Pod3-. Typical values are given in Table I for the two membranes operated a t axial velocities from 4 to 30 ft/sec. The notable feature of the results in Table I is that although the nominally iionrejecting Eastman membrane had a chloride rejection of -40%, virtually all other rejections mere substantially greater-Le., 74-83% for organic carbon, 92-98% for turbidity, 30-60% for Ca2T Mg2+ and 70-90% for P043-. 111most cases, all other rejections for the Xerojet membrane \\-ere substantially the same or somewhat greater than the chloride rejection. Previous studies (Sheppard and Thomas, 1970a,b) have shown that frequently a plot of log flus vs. log time produces a straight line-Le.,
+
+
2 1 1
2
5
10
2
5
2 3
IO2
TIME AFTER PRIMARY SEWAGE ADDITION (hrl
Figure 3. Effect of axial velocity and time on flux from aerojet 6% permeation cellulose acetate membrane at 1000 psig
,
1c
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2 5 102 Jo, I N I T I A L FLUX (gol/fI2 doy)
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Figure 4. Effect of initial flux and axial velocity on flux decline of cellulose acetate membranes in treatment of primary sewage effluent
straight line relationship of Equation 1 is followed quite well for times from 21/2t o 200 hr after introduction of primary sewage. Figure 4 is a plot of the value of b as a function of reference flux, J,, which was taken to be the flus 1/2 hr after initial startup with city water feed. Although only two membranes mere compared in this study, results for tests a t velocities from 6.4 to 30 ft/sec are not inconsistent with a l//2 power dependence of b on reference flus, J,. Based on this square root dependence, values of b/dx are shown plotted vs. axial velocity in Figure 5 . The open points are high-flus (Eastman) membrane results, and the half-filled points are the low-flus (Xerojet) membrane results. Although there is considerable spread in the data [as also observed in earlier studies using high-fouling potential untreated lake water as feed (Sheppard and Thomas, 1970a)], i t is clear that velocities in excess of 10-15 ft, see are required to obtain small values for the flus decline parameter, b. Discussion
where J Lis flus a t time, t; J , is the reference flus a t time, t,; and b is the slope of the straight line. The present results with primary sewage plant effluent as feed followed such a relationship when initial time was taken as the time when feed was switched from city mater to sewage plant effluent. Figure 3 shows results typical of those obtained in all our tests; the
The most significant result of these studies is the unequivocal evidence that the rate of flux decline is niarkedlj- different depending on whether the axial velocity is above or below a certain critical or threshold velocity. Below this velocity, flux decline is severe, and above this velocity flu\: decline is only moderate. Based on the marked reduction in rejection Ind. Eng. Chem. Process Des. Develop., Vol. 11, No. 3, 1972
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the equation can be rearranged to give
10-1
5
5 3
x
10-3
1
2
5
io
20
40
A X I A L VELOCITY
Figure 5. Effect of axial velocity on flux decline of cellulose acetate membranes in treatment of primary sewage effluent
observed a t low velocities, i t is postulated that below the critical velocity, particle or floc deposition occurs and that sufficient solids accumulate on the membrane surface to markedly increase concentration polarization (Sheppard and Thomas, 1971) as well as to cause a pronounced decrease in flux. If this is correct, then the critical velocity of this study may be related to the minimum transport velocity observed in transport of flocculated solids in impermeable pipes (Thomas, 1961). (The minimum transport velocity may be defined as that velocity required to prevent the accumulation of a layer of stationary or sliding particles on the bottom of a horizontal conduit.) Based on dimensional considerations, Thomas (1961) suggested that, for particles with diameter smaller than the viscous sublayer and which follow Stokes' law, the friction velocity for the minimum transport condition, ( u * ) ~ *may , be related to properties of the particulate matter by
where b' is not necessarily equal to b, and the primary variables v and u*, have been separated from those having only a secondary effect. Based on the limited data available, a = 0.3 and d = 110 when v and u*, are in cm/sec. If flocculated particles are primarily responsible for the particulate fouling and their size depends on velocity in the manner described above, then there is only a 15% uncertainty in the value of d owing to floc size variation in the present tests. This is substantially smaller than the experimental uncertainty of 1 4 0 % estimated from the results of Figure 2. Additional studies are required to obtain values of d for different feeds but based on experience with the minimum transport correlation (Thomas, 1961), it is believed that the value of a determined in this study should, in general, be valid for a variety of feeds. Since Equation 3 is formulated in terms of friction velocity, i t should be useful for predictive purposes for a variety of channel shapes, requiring tests a t only one flux to ascertain the value of A . Because of the complex nature of the constituents in polluted surface water feeds, it is difficult to envision a single mechanism for describing the flus decline with time. At the simplest level it would seem necessary to consider both buildup of a layer on the surface of the membrane as well as deposition of material within pores in the membrane. Although construction of a general fouling theory seems premature a t this time, it seems worth pointing out that apparently buildup of surface layers is minimized a t velocilies above the critical. If this is so, then the present high-velocity flux decline may represent fouling owing primarily to deposition of material within the membrane pores. From a cross-plot of the results of Figure 4, i t appears that a t high velocity the value of b (the exponent in Equation 1) assymptotically approaches a u-l'' dependence. Combining this with the flux dependence also shown in Figure 4 indicates that a t high velocities:
where CT is the t'erminal settling velocity of a particle (or floc) of diameter D,,and Y is the kinematic viscosity. K and b are to be determined from theory or experiment; reevaluat'ion of Thomas' (1961) data indicate K 'V 0.01 and b = 3 when t'he pipe wall is not' permeable. Characterization of the particle or floc size of the primary sewage effluent was beyond the scope of this sbudy. Based on studies elsewhere (Rickert and Hunter, 1967), it seems likely t'hat the particles are less than 10 pm in diam and further that' floes will be disrupted by the high shear conditions close to the membrane to give floc diameters of the order of 1 pm (Thomas, 1964). On this basis the settling velocit'y will be less than 10-4 cm/sec, more than an order of magnit'ude smaller than the normal velocity to the wall in this study (a flux of 50 gal/ftZ.day is equivalent, to a velocity of 2.3 X cm/sec). Hence t,he terminal settling velocity in Equation 2 can be replaced by the flux, v. Studies cited by Thomas (1964) indicate that in t,he high shear region near a surface, floes are disrupted and that even if disruption does not produce ultimate particles, floc size varies roughly as (u*)-'I2. For the conditions in the present study, this would mean, a t most, only a 20% change in floc in Equation 2 and size. Thus, after s u b d h t i o n of 2) for defining the critical or threshold friction velocity as u*,,
rT
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Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
(
A log flus
A log time
)
=
b
0:
(~)'iz
(4)
Although the mechanistic significance of this is not clear, it should provide a useful scaling 1aTv for design of equipment using high-flux membranes as they are developed in the future. Conclusions
Severe flux decline (observed a t low axial velocities) in hyperfiltration of sewage plant effluent with cellulose acetate membranes may be ameliorated by operation a t axial velocities in escess of 10 to 15 ft/sec when the initial flus is 40-150 gal/ft2. day. K i t h dynamic membranes, which frequently have initial fluxes of several hundred gal/ft2.day, Kraus (1970) concluded that velocities above 20 ft/sec are required to prevent unacceptable rates of flux decline ivhen the feed is primary sewage plant effluent. Although ayial velocities of 10-15 ft t see are considerably greater than are now common practice in brackish ater desalination ith cellulose acetate, economic optimization calculations (Thomas et al., 1970) indicate that with cellulose acetate-type membranes, optimum axial velocities (to give minimum water cost) are from 10 to 20 ft/sec for initial fluxes in the range 80-150 gal/ft2.day. Thus high-flus cellulose acetate membranes tailored to meet requirements for sewage treatment or possibly
nonrejecting (such as Eastman HT-00 used in this study on which dynamic membranes can be formed from sewage constituents) may find a place in treatment of sewage plant effluents where incomplete salt rejection can be tolerated and where substantially greater removal of turbidity and organic carbon is desired. Acknowledgment
The authors wish to acknowledge the support and encouragement of K. A. Kraus and the assistance of J. R. Love in carrying out these tests. literature Cited
Bregman, J. I., Environ. Sci. Technol., 4, 296 (1970). Hauck, A. R., Sourirajan, S., ibid., 3, 1269 (1969). Kraus, K. -I., Water Pollution Control Research Series ORI)-17030EOHOl/70, U.S. Dept. of the Interior, Federal Water Quality Administration, 1970. Merten, U., Bray, D. T., “Proc. 3rd I n t . Coni. hdvan. K a t e r Pollut. Re..,” Xunich, Germany, Vol. 3, pp 315-31, K a t e r Pollution Control Federation, TTashington, DC, September 1966.
Okey, R. W., Staverman, P. L., Proc. Symp. Xembr. Process. Ind., pp 127-56, Birmingham, -IL, X a y 19-20,1966. Rickert, D. A, Hunter, J. V., J. Water Pollut. Contr. Fed., 39, 1475 (1967). Savage, H. C., Bolton, X. E., Phillips, H . O., Kraus, K. .I., Johnson, J. S., Jr., V a t e r Sewage If-orks, 116 (3), 102-6 (1969). Sheppard, J. D., Thomas, D. G., Appl. Polym. Symp., Yo. 13, A. F. Turbak, Ed., 121-38 (197Oa). Sheppard, J. D., Thomas, D. G., Desalination, 8 , 1-12 (1970b). Sheppard, J . D., Thomas, D. G., AIChE J . , 17, 910-15 (1971). Thomas, D. G., ibid., 7,423-43 (1961). Thomas, D. G., ibid., 10,517-23 (1964). Thomas, D. G . , Griffith, IT. L., Keller, R. lI.,Desalination, 9, 33-50 (1971). RECEIVED for review September 22, 1970 ACCEPTED January 24, 1972 Research sponsored jointly by the K a t e r Quality Office, Eiivironmental Protection Agency, and the U.S. Xtomic Energy Commission under contract with the Union Carbide Corp.
Design of Cross-Flow Cooling Towers and Ammonia Stripping Towers Walter J. Wnek and Richard H. Snow1 I I T Research Institute, 10 TT’est 35th St., Chicago, I L 60616
A method of designing cross-flow cooling and ammonia stripping towers is presented which avoids the numerical analysis previously required for cooling tower design. Approximate analytical solutions are obtained for the simultaneous equations of conservation of energy and mass. The results agree with examples from the literature obtained b y a less general finite-difference method, and also with data from a pilot ammonia stripping tower. Equipment and operating cost correlations are presented, and also a method to optimize ammonia stripping tower design geometries. A sample calculation for a 1 -million g p d tower treating waste water from a typical municipal treatment plant shows that a tower 57 f t high with packing only 7 f t thick i s optimum to reduce ammonia concentration b y 40 : 1 . Reduction of 8 : 1 requires a 32-ft tower. Whether such towers can b e successfully operated, even if liquid distributors are added, has not been demonstrated; however, the results show that a short, wide tower will not perform as specified. The capital cost for the example tower i s $ 1 million, while the total unit cost i s 4 cents/l000 gal. Such high costs make the use of stripping towers unlikely for waste water treatment, although they cannot be ruled out because other feasible methods have so f a r not been found. The design methods are applicable to other cross-flow stripping systems, such as odor control.
C
rosa-flow towers for cooling and stripping operations are becoming increasingly important because of today’s more demaiiding situations. For example, it has been suggested that cross-flow towers may be more economical than countercurrent. towers for stripping ammonia from liquid waste n-here the concentration is on the order of mg, 1. or ppin. This is so became cros-flow towers allow use of a larger volume of air a t a lower fan power consumption. For t,his reason, the To whom correspondence ,\hould be addressed.
ammonia stripping tower built a t Lake Tahoe is of cross-flow design (Slechta and Culp, 1967). hlt,hough cross-flow towers offer a number of advantages over countercurrent towers, their use has been hampered b y a lack of adequate design procedures. Previous design procedures involve a numerical analysis using finite differences with a digital computer. Schechter and Kang (1959), extending the work of Zivi and Brand (1956), used this approach and presented set’s of design curves for cooling towers. Unfortunately, it is difficult to use their results for design purposes Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
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